1. Technical Field of the Invention
The present invention relates generally to skin friction meters and more particularly to a reflection type skin friction meter for both flight and wind tunnel applications.
2. Discussion of the Related Art
The determination of skin friction is very important in fluid mechanical studies or aerodynamic effects. Several methods have been disclosed and include a dual beam interferometer disclosed in U.S. Pat. No. 4,377,343 to Monson. In this patent, each beam is read by a light detector to measure the thickness of an applied oil film at two different points in order to generate a thickness history indication of skin friction by counting the number of interference fringes. A clean, particle free flow is required since the presence of, e.g., a dust speck in the oil film can disrupt the required continuous counting. Also, this method is very sensitive to vibration, requires a relatively large laser capable of producing coherent light in order to obtain good interference fringes, is not easily adaptable from wind tunnel test to flight conditions, requires an optically smooth surface for the interferometer, and has a complicated optical system to produce the necessary two beams.
Another proposal for measuring skin friction is disclosed in U.S. Pat. No. 3,714,824 to Bush. A ring shaped element is placed in a seat in an upright container and subjected to an air flow. The frictional forces of the flow will overcome gravity and lift the ring off its seat at a point corresponding to the specific skin friction. This method only permits the determination of large area surface averaged skin friction of a particular ring mounted only vertically, precluding straightforward adaption to a variety of surfaces and/or velocities. Also, the method cannot be adapted to flight conditions.